This is related to my prior filed application, entitled ‘Container Having Passive Controlled Temperature Interior, and Method of Construction,’ Ser. No. 10/278,662, filed Oct. 23, 2002, and my prior filed application, entitled ‘Container Having Passive Controlled Temperature Interior,’ Ser. No. 10/411,847, filed Apr. 11, 2003.
The shipment of temperature-sensitive goods is extremely difficult when the shipping container itself is not independently temperature-controlled; ie, does not have an independent power source for maintaining interior temperatures within close parameters. Of course, if it is merely desired to maintain an object to be shipped at a nominally cooled temperature—relative to the ambient exterior temperature—a common practice is to pack a shipping container with ice, and hope that the ice will remain in a frozen state during transit so that the object shipped will arrive at its destination still cooled below ambient temperature. This can be an adequate technique for shipping objects where temperature control is not critical. However, even in this case, the temperatures at different points inside the shipping container will vary widely, with parts of the interior of the container becoming quite cool and other parts of the interior warming to various degrees, depending on time and the distance and spatial relationship of the shipped object to the cooling ice which remains in the container.
In shipping objects for which the ambient temperature is expected to be cooler than the desired temperature for the object, the common practice is to place the warmed object inside a container having insulated walls, and then to hope the shipping time is shorter than the time for the heat inside the container to escape through the insulated walls.
The present invention, and my prior-filed applications, cover inventions which utilize three important principles to construct a new and novel construction, and method of using, to provide improved temperature controls inside shipping containers. First, a characteristic of any phase change material, ie., a material capable of converting from a solid to a liquid, or vice versa, is that during the conversion process the material maintains a constant temperature, referred to as the “melting point;” until the entire volume of the material has been converted to the other state.
Second, a given volume of phase change material requires a certain cumulative transfer of heat, either into the material or out of the material, known as the “heat of fusion,” usually measured in Btus/lb. (British thermal units per pound by weight).
Third, the rate at which Btus can be absorbed into the material or transferred outside the material determines the total time that a conversion process requires, ie., the total time that the temperature of the material, and the container in which it is contained, will remain constant at the melting point. This total time can be controlled by enclosing the material and its container in an insulated enclosure which limits the heat transfer rate to a desired quantity.
Different phase change materials may have different melting points and different heats of fusion, so that it is possible to construct a container having an internal constant melting point temperature at 0°, or at some other control temperature which may be desired. For example, water has a melting point of 0° C. and a heat of fusion of 144 Btus/lb, deuterium oxide (heavy water) has a melting point of 4° C. and a heat of fusion of 136 Btus/lb. Many commercially-available phase change materials have melting points over a wider range than water, but have heats of fusion ranging around 70/Btus/lb. This means that greater or lesser volumes of a phase change material may be required, depending upon the desired melting point temperature. Another factor of concern is material cost; the cost of deuterium oxide is about $200/lb., while the cost of many commercially-available materials is only several dollars per pound. Finally, yet another factor is shipping cost, where the size and weight of the container can greatly affect the cost to ship products.
My prior filed patent applications disclose several alternative constructions for containers having passive, reliable and relatively inexpensive structures for protecting highly temperature-sensitive products and materials during shipment and short-term storage. Such products and materials are usually fairly high in value and may be extremely temperature-sensitive. Some examples of such products or materials are blood shipped or carried to remote battle zones, sensitive pharmaceuticals shipped between plants or to distributors, HIV vaccines shipped to third world countries, and medical instruments shipped to, or kept in readiness at, remote stations or in emergency vehicles. In such cases the ambient temperatures may vary widely, from extremely hot shipping facilities in the southern states to receiving points in cold, mountainous regions of the world in midwinter.